One thing i think needs to be clear is why people want to say vitality is better. While toughness is useful the amount of damage reduction doesn’t change based on how much health you have. Because of this it is no more useful on a necromancer than any other class.
Vitality on the other hand increases your health pool. which increases life force due to it being based on your health. Because of this vitality is 1.69 times more effective on a necromancer than other classes.
This doesn’t mean it is automatically better than toughness, but is why instinct would lead some to believe that without testing it. The biggest thing to keep in mind is that your specific situation and what kind of content you are playing will have a big effect on what stat is most useful.